1. Field of the invention
This invention relates to a plasma display panel, referred to hereinafter as a PDP, of a matrix display of an AC driving type, and more particularly, relates to a surface discharge form, in which the discharge is generated along the display surface.
2. Description of the Related Arts
PDP is a thin matrix display panel of self-luminous type of a display, and has been widely employed in the application to television pictures and computer monitors upon the achievement of the color display. PDPs have also been remarkable for the large size flat display device, such as for the use in the HDTV, High Definition Television.
A memory effect is utilized in sustaining the lighting state of the cells of the display elements in the PDP of the matrix display type. The AC type PDP is constituted such that the structure has a memory function by coating an insulating material upon electrodes. In the display of the AC type PDPs, a line sequential addressing is performed to accumulate wall charges only in the cells to be luminous, i.e. to be lit; next, sustain voltages of alternating polarization are simultaneously applied onto the entire cells. The sustain voltage is of a predetermined voltage lower than the discharge firing voltage. In the cells having the wall charges therein the sustain voltage superposed with the wall voltage makes the effective voltage applied to the cell exceed the firing voltage resulting in generation of the discharge. Shortening of the sustain voltage application interval provides a visually continuous lighting state.
AC type PDPs of surface discharge form have been commercially on market as color display devices. The surface discharge form is such that pairs of sustain electrodes which are arranged in parallel on a single substrate and one of the pair alternately becomes anode and cathode during a sustain period, that is a display period. The surface discharge form PDP allows us to expect a long operating life by arranging fluorescent material layers for color display on an opposing substrate so as to reduce electron bombardment thereon during the discharge.
FIG. 10 schematically illustrates an internal structure of a prior art PDP 90. FIG. 11 schematically illustrates a luminous strength distribution along the aligning direction of the prior art sustain electrodes.
In PDP 90, on an inner surface of a front glass substrate 91 are arranged pairs of sustain electrodes, that are a first and a second electrode 93 & 94 for each single line of the display matrix. These sustain electrodes 93 & 94 are insulated from a discharge space 99 by a dielectric layer 96. Upon a surface of dielectric layer 96 is provided a protection layer 97 formed of a material having a high secondary emission coefficient. Upon an inner surface of a back glass substrate 92 are arranged the third electrodes 95, that are address electrodes, for each row orthogonal to sustain electrodes 93 & 94. There is provided a fluorescent material layer 98 so as to cover the back glass substrate 92 including the upper surface of address electrodes 95. The type of the fluorescent material layer 98 arranged on the back substrate is called a reflection type. The type of the fluorescent material layer 98 is arranged on the front substrate is called a transmission type. The reflection type is more advantageous than the transmission type in the brightness and the view angle characteristics because the lighting surface of the fluorescent material layer 98 can be directly observed.
First sustain electrode 93 is a composite electrode in which an electrically conductive transparent film 931 is stacked with a metal film 932 as a supplemental conductor which is narrower in the width than electrically conductive transparent film 931, and extends along the line direction. Second sustain electrode 94 is also a stack of an electrically conductive transparent film 941 and a metal film 942, similar to sustain electrode 93. Widths of transparent films 931 & 941 are chosen according to the cell size so that an appropriate electrode spacing is secured between the adjacent lines and the surface discharge can extend widely within the cell. Widths of metal films 932 & 942 are chosen depending on the length so as to secure the electrical conductivity more than the allowable lowest limit. Electrode spacing S2 between the adjacent lines is called an inverse slit.
In the display using PDP 90 a line-sequential addressing is performed. In lighting a cell, address electrode 95 and second electrode 94 are respectively biased appropriately so as to cause an opposing discharge therebetween along the substrate thickness direction so that the surface of the dielectric layer 96, where protection layer 97 is included therein, is appropriately electrically charged, which is called a wall charge. After the addressing operation to determine the lighting/non-lighting of each cell by thus generated wall charge, sustain voltages are applied to first and second sustain electrodes 93 & 94 where the polarity of the applied potential difference is alternately changed, whereby the surface discharges are cyclically generated in the display cells in which the wall charges have been formed. Fluorescent material layer 98 emits a predetermined visible light by being locally excited by an ultraviolet light generated in the surface discharge. The light transmitted through the glass substrate becomes the display light.
As shown in FIG. 11, the luminous strength of the emitted light within each cell becomes maximum at the center of the surface discharge gap S1, which is referred to as a discharge slit, and becomes smaller as leaving along the row direction from the center. In the prior art technique, the metal films 932 & 942 are arranged so as to shift to an extreme end from the surface discharge gap S1, i.e. the side near to the inverse slit S2, of transparent films 931 & 941 in order to minimize the decrease in the luminous strength.
It has been constantly required that the driving voltages of PDPs be lowered from the view points of power consumption, thermal matters and minimization of the driving circuits.
However, on the other hand the display has been required to be finer; accordingly, the cell size must become smaller. In a smaller cell, the discharge firing voltage rises due to the suppression of the movement of the charged particles. This fact is considered that the plasma confined into a small area is likely to cause the reunion to the wall charges, and the amount of the wall charges are so insufficient as to maintain the effective voltage.
In the prior art structure, it has been a problem in that though the shadowing by the metal films 932 & 942 can be kept minimum the luminous efficiency, i.e. brightness/power consumption, is decreased as the cell size is decreased.